Integrated drive generators have been in use for many years in generating electrical power on airframes. An integrated drive generator functions to produce three phase 400 Hz. alternating current when driven by a variable speed power takeoff from the airframe propulsion engine. The integrated drive generator is contained in a single case which contains a constant speed drive transmission which converts the variable speed shaft input from the power takeoff from the airframe propulsion engine to a constant speed drive for driving an alternating current generator mounted within the case of the integrated drive generator. The constant speed drive transmission includes critical elements which require pressurized oil during the generation of alternating current by the integrated drive generator. These units include a hydraulic pump and motor, differential, cooling for the main generator and lubrication of the rotor of the main generator. All of these elements operate in a conventional fashion as is known in the art which does not form part of the present invention.
FIG. 1 illustrates a block diagram of a prior art oil supply system for an integrated drive generator of the type manufactured by the assignee of the present invention. The integrated drive generator 10 has a case (not illustrated) which collects oil in an oil sump 12 located in the bottom of the case. The oil sump 12 supplies oil which is pressurized and applied to various parts of the integrated drive generator as described below. A scavenge pump 14 pressurizes oil from the oil sump 12 and applies it to a filter and deaerator (not illustrated). The output of the deaerator, which contains oil of higher quality than that pumped by the scavenge pump 14, is applied to the intake of charge pump 16. A charge pump 16 functions to pressurize the oil with a head such as 250 psi. The output of the charge pump 16 is applied to an oil circuit 18 which contains a plurality of parallel branches 20, 22, 24, 26 and 28. Branch 20 supplies pressurized oil to a hydraulic pump and motor 30 which is a conventional part of the constant speed drive transmission contained in the integrated drive generator. Branch 22, which contains a restriction 32, provides a bypass around the hydraulic pump and motor to permit the bypassing of oil around the hydraulic pump and motor instead of requiring that all oil being applied to charge relief valve 34 must flow through the hydraulic pump and motor 30. Charge relieve valve 34 controls the output pressure of oil in the oil circuit 18 by shunting oil back to the intake of charge pump 16 when the pressure of oil outputted by the charge pump is above the limit at which the charge relief valve opens. Branch 24 applies pressurized oil to differential 36 which is part of the conventional constant speed drive transmission contained within the integrated drive generator. Oil from the differential 36 is outputted to the main sump 12. Branch 26 applies pressurized oil to the back iron of the main generator stator 38. The back iron of the stator 38 contains channels through which oil flows to cool the main generator in a conventional fashion. Oil is outputted from the oil cooling circuit of the stator 38 to oil sump 12. Pressurized oil in branch 28 cools the main generator rotor 40 and is applied to the bearings of the rotor to provide lubrication for the bearings rotatably supporting the rotor. Oil is outputted from the bearings of the main generator to the oil sump 12.
While an oil circuit generally similar to that illustrated in FIG. 1 has worked satisfactorily in airframes, it has the disadvantage of requiring a scavenge pump 14 and charge pump 16 which are sized to satisfy the parallel oil flow requirements of the hydraulic pump and motor 30, differential 36, main generator stator 38 and main generator rotor 40. As a consequence of the oil flow requirements for supplying pressurized oil to the parallel oil circuits of the hydraulic pump and motor 30, differential 36, main generator stator 38 and main generator rotor 40, the overall capacity of the charge pump must be rated to exceed the cumulative maximum oil capacity which may be drawn by each of the aforementioned units within the integrated drive generator. Furthermore, in order to preclude any possibility of starving the charge pump 16 of oil, the scavenge pump 14 must be sized to provide a larger oil flow to the intake of the charge pump, such as 150% of the rated output of the charge pump. As a result, the parallel oil flow circuits 20, 24, 26 and 28, which respectively provide oil to the hydraulic pump and motor 30 differential 36, main generator stator 38 and main generator rotor 40, require the scavenge pump 14 and charge pump 16 to be large enough to satisfy the aforementioned oil flow requirements.
Any reduction in the overall rated output of the charge pump results in a weight savings in the integrated drive generator in that the charge pump may be downsized and further the scavenge pump may be downsized even further as a result of its having a rated output higher than the rated output of the charge pump. Reduction in pump size also enhances the overall efficiency of the integrated drive generator.